[rust] fix: free-form object query parameters reference a nonexistent models::serde_json

[wiebren]

A free-form object parameter — type: object with no properties — is typed
Option<models::serde_json::Value> in every generated api module, but models re-exports no
serde_json, so the crate fails to compile with one E0433 per operation that uses one:

parameters:
  - in: query
    name: filter
    schema:
      type: object

pub async fn list_things(configuration: &configuration::Configuration, filter: Option<models::serde_json::Value>, ...)

error[E0433]: failed to resolve: could not find `serde_json` in `models`

Free-form body parameters are typed serde_json::Value and compile fine; only query, path,
and header parameters are affected. All four rust client libraries (reqwest, reqwest-trait,
hyper, hyper0x) share the bug.

The cause

The api templates qualify a parameter's type with models:: unless it is a string, uuid,
primitive, or container — rust/reqwest/api.mustache and the same pattern in the other three
libraries:

{{^isString}}{{^isUuid}}{{^isPrimitiveType}}{{^isContainer}}models::{{/isContainer}}{{/isPrimitiveType}}{{{dataType}}}{{/isUuid}}{{/isString}}

A free-form object maps to serde_json::Value (typeMapping.put("object", "serde_json::Value")),
which lives outside models. Body parameters escape because
DefaultCodegen.updateRequestBodyForObject marks free-form bodies isPrimitiveType = true,
while fromParameter sets only isFreeFormObject — so the prefix lands on non-body parameters.

The fix

RustClientCodegen.postProcessOperationsWithModels now marks free-form, non-container
parameters as primitive — the treatment free-form bodies already get — right next to the
existing isAnyType special case. One codegen change covers all four libraries' templates.

Non-container matters: a free-form schema with additionalProperties is typed
HashMap<String, …>, is already exempt from the prefix as a container, and must keep its
serde_json::to_string query serialization since HashMap has no Display.

Wire behavior is unchanged: the query serialization branch for these parameters is chosen by
isObject/isMap/isDeepObject, not isPrimitiveType, and the required-parameter path
always used .to_string() (for serde_json::Value, Display is its JSON serialization).

Tests

RustClientCodegenTest#testFreeFormObjectQueryParam, generating from the new fixture
3_0/rust/free-form-object-query-param.yaml, locks in:

• filter: Option<serde_json::Value> and no models::serde_json anywhere;
• type: object with typed additionalProperties staying HashMap<String, String>;
• additionalProperties: true (free-form and a map) staying
  HashMap<String, serde_json::Value> with serde_json::to_string serialization.

It fails without the main/ change.

Verified by compiling generated clients

Clients generated from a spec with optional, required, and map-typed object query parameters:

library	cargo build
reqwest (sync)	clean
reqwest (async)	clean
hyper	clean
reqwest-trait	clean for the free-form parameters this PR touches

Two adjacent breakages exist identically on master and in v7.15.0, i.e. before this change,
and are left out of scope: reqwest-trait calls .to_string() on map-typed query
parameters (which this PR does not touch), and the exploded style: deepObject free-form
branch iterates a serde_json::Value (.len()/.iter()), which does not compile either way.

PR checklist

• Read the contribution guidelines.
• Built the project and updated samples: ./bin/generate-samples.sh ./bin/configs/rust-*
  regenerated all 39 rust configs with zero diffs (no sample spec has a free-form object
  parameter outside a body); ./bin/utils/export_docs_generators.sh produced no diff.
• Technical committee (Rust): @frol @farcaller @richardwhiuk @paladinzh @jacob-pro @dsteeley

---

Generated with Claude Code

---

Summary by cubic

Fixes generated Rust clients failing to compile when an operation uses a free-form object query, path, or header parameter. These were typed models::serde_json::Value, which doesn't exist; they're now serde_json::Value, matching free-form body parameters.

• One codegen change covers all four Rust generators (reqwest, reqwest-trait, hyper, hyper0x).
• Free-form objects with additionalProperties stay HashMap-typed and keep their existing query serialization; no wire behavior changes.
• Adds a regression test covering plain, typed-map, and additionalProperties: true free-form object query parameters.

^{Written for commit 1a9f875. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}